Billing after 2 years??

I have been a customer of virgin mobile but moved to another supplier one and a half years ago. My number has been ported from Virgin. It is in use with my new supplier.<br> <br> After one year I received a call from Virgin to tell me that I still owed them money, which I payed in November 2015. giving them the benefit of the doubt. Now I received another letter from MBD Legal Collections telling me I still owed money to Virgin and they now have the rights to collect the money I owe, a new amount of R120.<br> <br> I want to know how my telephone number can accumulate a bill at two different suppliers?? I have not been with virgin for nearly two years now, where do they get that amount??<br>